The subventricular zone (SVZ) of the lateral ventricles and the hippocampus of the mammalian brain contain populations of multipotent neural stem cells (NSCs). These cells continually divide, asynchronously, resulting in a replacement stem cell and a proliferative progenitor. Following additional division and migration, proliferative progenitors differentiate towards multiple neural lineages. It has been hypothesized that NSCs participate in neurogenesis throughout the life of the organism and contribute to maintaining the health of the brain and/or its functions, including memory. It is possible that NSCs undergo age- related changes that may lead to dysfunction or contribute to pathologies including Parkinson's or Alzheimer's diseases. The objectives of this proposal are to define changes in the presence, cell cycle parameters and function of the NSC and proliferative progenitor populations during aging in the brain using the mouse as a model. Where possible, such changes will also be assessed in human samples. Aim 1: A double nucleotide analog labeling method, and immunological detection of the DNA replication licensing factor MCM2 will be utilized to define replication competent long-term label retaining cells (putative stem cells) and proliferative progenitors within the SVZ of mice of different ages. Cell cycle parameters for each population of cells will be defined from these data. This set of experiments will define changes in the stem cell and proliferative progenitor concentrations and cell cycling times as a function of age and serve as a baseline for additional studies. Additionally, we will test the utility of expression of the DNA polymerase subunit proliferating cell nuclear antigen (PCNA) as a surrogate for nucleotide analog incorporation in defining the proliferative progenitor population. Finally, changes in MCM2 and PCNA expression in the SVZ and hippocampal regions of human brain sections will be assayed, immunologically, as a function of age. Aim 2: Using methods described above to monitor NSC and proliferative progenitor parameters, we will determine if changes in the properties of these cells during aging are cell autonomous or dependent on the environment of the aging brain using NSC transplantation techniques. Aim 3: Transgenic mouse models will be constructed in which the MCM2 gene is marked by incorporation of either enhanced green fluorescent protein (EGFP) or a tamoxifin inducible Cre-estrogen receptor fusion. These models will allow: 1) determination of whether all quiescent neural stem cells express MCM2 or, alternatively, if a sub-population MCM2 negative cells exists and 2) the fate of NSC populations to be followed, respectively. The ability to monitor the fates of neural stem cell progeny in vivo will allow us to determine if age- related changes in the differentiation potentials of these cells occur.